A pulse-width modulated signal amplifier is known from U.S. Pat. No. 4,021,745. In this patent, a low-pass filter is used to filter out amplified low frequency signals. But a low-pass filter cannot suppress very well the carrier frequency dominating the frequency spectrum, for which reason a clean-cut separation of the information carrying signal from any perturbing signals, and therefore also a high signal quality, is not attainable.
There is further known from the IEEE Journal of Solid State Circuits, February, 1969, pages 34-35, a circuit for an induction heater in which the phase angle of an inductive load is detected, and the frequency of a signal source controlling a switching amplifier is regulated. This circuit is not, however, utilized for a pulse-width modulated signal amplifier; furthermore, regulation of the frequency of the signal source is not accomplished in order to suppress an interferring signal, but to adopt the circuit to any load changes.